Photorefractive crystalline materials such as iron-doped lithium niobate (Fe:LiNbO.sub.3) respond to exposure to light in such a way as to cause a charge redistribution which can later diffract incident light. Thus, if the initial exposure creates a hologram, the latter can be reconstructed and recovered.
To date, the utility of lithium niobate as a holographic data storage medium has been severely limited by the lack of a rapid, selective, and reversible storage (write) procedure which is compatible with non-destructive data recovery (read).
D. L. Staebler and J. J. Amodei ("Thermally fixed holograms in LiNbO.sub.3," Ferroelectrics, 3, p. 107, 1971) demonstrated that holograms generated within Fe:LiNbO.sub.3 could be thermally fixed by heating the Fe:LiNbO.sub.3 to roughly 100.degree. C. for at least 30 minutes after the holograms had been generated. While this technique is reversible (by heating the Fe:LiNbO.sub.3 to greater than 170.degree. C. for approximately 30 minutes) and does permit non-destructive data read out, it is slow, results in low holographic diffraction efficiency, and is non-selective (i.e., all holograms multiplexed within a given volume of Fe:LiNbO.sub.3 are fixed and erased, en mass).
Subsequently, D. L. Staebler, W. J. Burke, W. Phillips and J. J. Amodei ("Multiple Storage and Erasure of Fixed Holograms in Fe-doped LiNbO.sub.3," Applied Physics Letters, 26, p. 182, 1975) demonstrated that volume holograms generated within Fe: LiNbO.sub.3 could be thermally fixed by heating the Fe:LiNbO.sub.3 to approximately 160.degree. C. while the holograms were being generated. While this technique resulted in high-diffraction efficiency holograms that could be non-destructively read-out, it necessitated an inconvenient physical rotation of the Fe:LiNbO.sub.3 crystal during read-out in order to compensate for hologram read-out angles which changed slightly upon cooling the Fe: LiNbO.sub.3 crystal to room temperature.
D. von der Linde, A. M. Glass and K. F. Rogers ("Multiphoton photorefractive processes for optical storage in LiNbO.sub.3," Applied Physics Letters, 25, p. 155, 1974) describe a two-photon storage technique which is rapid (picosecond time scale), but which requires high-intensity laser beams (.about.10.sup.9 watts/cm.sup.2) for storage and relatively low intensity laser beams for non-destructive read-out. Such picosecond, gigawatt laser pulses are difficult to obtain at repetition rates (10-1,000 Hz) necessitated by practical data storage systems and, furthermore, may cause damage to critical system components (e.g., acousto-optic beam deflectors used for hologram multiplexing and spatial light modulators used for entering holographic data). Moreover, the requirement to use a low-intensity laser beam to avoid destructive read-out at the second harmonic frequency inevitably leads to reduced output data transfer rates.
Finally, D. von der Linde, A. M. Glass and K. F. Rogers ("Optical storage using refractive index changes induced by two-step excitation," Journal of Applied Physics, 47, p. 217, 1976) describe a two-step holographic storage process involving chromium-doped LiNbO.sub.3 which is, in principle, capable of non-destructive read (at the write wavelength) and selective optical erasure, and which requires peak storage laser beam intensities of only about 10.sup.7 watts/cm.sup.2. This approach, however, yields relatively small holographic diffraction efficiencies (compared with Fe:LiNbO.sub.3) which are not linear with storage exposure energy (important for retaining holographic dynamic range) and, most importantly, yields short storage times of only about 20 hours. Furthermore, this approach requires the use of specialized (Q-switched ruby and tunable dye) lasers which are complicated, unreliable, and expensive.
There is, therefore, a need for improved methods of fixing holograms in photorefractive storage media which will overcome the above limitations and disadvantages and, thereby, make possible practical read/write memories based thereon.